ar X iv : q ua nt - p h / 06 06 01 8 v 1 1 Ju n 20 06 Transfer of trapped atoms between two optical tweezer potentials

The spatial control of atoms, beyond their trapping in stationary potentials, has been continuously gaining importance in investigations of ultracold gases and in the application of atomic ensembles and single atoms for cavity QED and quantum information studies. Recent progress includes the trapping and control of single atoms in dynamic potentials [1, 2], the sub-micron positioning of individual atoms with standing-wave potentials [3, 4], micro-structured and dynamic traps for Bose-Einstein condensates [5, 6] and, as another example, the realisation of chaotic dynamics in atom-optics ”billiards” [7, 8]. The reaction of trapped atoms to dynamical variation of the trapping potential is one central aspect of these developments. Its understanding is important to design optimally the shape of the potential and its temporal variations which provide the desired control over the atoms and allow their manipulations. The question of efficiently steering atoms by dynamically variable light beams shares many similarities with the application of laser beams for optical tweezers, for manipulating microscopic objects such as beads or living cells [9]. In the context of quantum information processing, it is also related to recent developments towards position control of single trapped ions in complex, multi-segment ion traps [10]. We report on a particular case of manipulation of atoms by dynamic variation of light potentials formed by laser beams: the transfer of a cold cloud of rubidium atoms between two horizontal, intersecting, focused laser beams. The transfer happens by ramping the intensities in the two beams, which is done slowly enough to give all atoms time to adjust to the change. The measured signal is the amount of atoms remaining in the second beam after the first one has been fully switched off, i.e. the transfer efficiency. Our main finding is a peculiar dependence of the efficiency on the vertical distance be-